--- In
Homebrew_PCBs@yahoogroups.com, "Jeff" <jeff.heiss@...> wrote:
>
> I was suggesting not to remove the copper, but to remove the paint. After the paint is removed, place the board in etchant and etch. The copper will remain where the paint was not removed leaving the traces. Or is this what you meant when you said it would not work?
>
Cary was responding to both possibilities -- if you want to remove copper directly, EDM ∗could∗ do it, but 1) it would be slow, and 2) you would have to compensate for the erosion of the electrode -- ie, you can't just move X and Y, but you would also have to move Z, all while determining whether the spark gap is too small (shorting) or too long -- and if too long, in which axis it is too long. I'll add two other points to what Cary said: 3) EDM produces a certain amount of overcut, meaning that your electrode will have to be ∗smaller∗ than the desired cut you are trying to achieve. If you want your spacing to be 10 mils, you'll need to have your electrode be around 6 mils in diameter -- and don't forget that it can't just be the point of the electrode that size; it has to be enough length of the electrode to allow for the gradual erosion of the electrode. And the electrode needs to be stiff enough not to deform if/when it contacts the board in the process of fine-tuning the spark gap. 4) You would have to program your cuts in such a way that you always maintain the ground on the material to be removed up until the very last spark. IOW, you could not clear out an area after cutting its outline, because the area would then be isolated from the ground. So -- all of that to say that using EDM to remove copper from the board is theoretically possible, but it would be a poor choice.
If you want to remove paint, rather than copper directly, EDM cannot do it at all, since EDM depends on generating a spark between the electrode and the conductive metal to be removed. I suppose you could get around that by using a conductive paint, but then 1-3 above would apply, and you would add the new problem of electrically distinguishing between the paint and the copper -- I don't think there is any way you could spark all of the paint off without also sparking off at least some of the copper, and in any case, until the copper is also gone, your EDM would not be able to tell that it was finished with that spot. You could try to get around that by having it only spend a certain amount of time in each spot, but note that that is not a simple matter when the control circuitry is sensing the spark gap and adjusting the electrode to account for wear as well as for how much cutting has occurred.
Whew! Just trying to describe all the problems of using EDM for this purpose has made me tired.